So-called resonant converters have a resonant circuit, which can be a series or parallel or series-parallel resonant circuit. When configuring converters, one aim is to keep losses low. For example, resonant converters which comprise an LLC series-parallel resonant circuit having two inductances and one capacitance are well-known. Such converters have the advantage that energy-efficient operation with relatively low switching losses is possible.
Resonant LLC converters are well known for use within LED drivers. The converters can be configured or operated as a constant current source or a constant voltage source. A constant current source can be used to drive an LED arrangement directly, thus enabling a single stage driver. Constant voltage sources can be used, for example, for LED modules which have further driver electronics in order to ensure a corresponding power supply to the LEDs with a predetermined current from the output voltage provided by the constant voltage source.
The LLC converter comprises a switching arrangement (which together with the gate driving arrangement is generally referred to as the inverter) for controlling the conversion operation, and the switching is controlled using feedback or feedforward control, in order to generate the required output.
Another function implemented within a power converter which is supplied with mains (or other AC) power is power factor correction (PFC). The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit. A power factor of less than one means that the voltage and current waveforms are not in phase, reducing the instantaneous product of the two waveforms. The real power is the capacity of the circuit for performing work in a particular time. The apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.
If a power supply is operating at a low power factor, a load will draw more current for the same amount of useful power transferred than for a higher power factor.
The power factor can be increased using power factor correction. For linear loads, this may involve the use of a passive network of capacitors or inductors. Non-linear loads typically require active power factor correction to counteract the distortion and raise the power factor. The power factor correction brings the power factor of the AC power circuit closer to 1 by supplying reactive power of opposite sign, adding capacitors or inductors that act to cancel the inductive or capacitive effects of the load.
Active PFC makes use of power electronics to change the waveform of the current drawn by a load to improve the power factor. Active PFC circuits may for example be based on buck, boost or buck-boost switched mode converter topologies. Active power factor correction can be single-stage or multi-stage.
In the case of a switched mode power supply, a PFC boost converter is for example inserted between the bridge rectifier and the mains storage capacitor. The boost converter attempts to maintain a constant DC bus voltage on its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Another switched-mode converter inside the power supply produces the desired output voltage or current from the DC bus.
Due to their very wide input voltage range, many power supplies with active PFC can automatically adjust to operate on AC power for example from about 110 V to 277V.
Power factor correction may be implemented in a dedicated power factor correction circuit (called a pre-regulator), for example placed between the (mains) power supply and the switch mode power converter which then drives the load. This forms a dual stage system, and this is the typical configuration for high power LED applications (for example more than 25 W). The power factor correction may instead be integrated into the switch mode power converter, which then forms a single stage system.
This invention relates in particular to a two-stage circuit having an isolating PFC front end and a non-isolating back end output stage. The front end is for example a resonant LLC converter and the output stage may be a buck converter. The LLC front end is capable of handling a wide AC input voltage range if the power conversion profile essentially corresponds to that required for a high power factor operation.
An example of a resonant DC/DC converter is shown in FIG. 1 in general form.
The circuit comprises a DC input terminal 2 (labeled B in FIG. 1 and all other figures) for receiving a rectified output, and which connects to a half-bridge having a first power switch 28 and a second power switch 30. The first switch and the second switch can be identical, and the half-bridge may for example be operated at a symmetrical 50% duty cycle. These switches can be in the form of field-effect transistors.
A resonant tank circuit 25 is connected to an output node, labeled X in FIG. 1 and all other figures between the two switches 28, 30.
Each switch has its timing of operation controlled by its gate voltage. For this purpose, there is a control block 31 (including a low voltage supply). The block 31 receives a control signal CTRL for controlling the gate voltages and a supply voltage SUP. Feedback (not shown) is used to determine the timing of the control of the switches 28, 30. The output of the resonant tank circuit 25 connects to a rectifier 32 and then to the load, in parallel with a smoothing capacitor CDC.
During operation of the converter, the controller 31 controls the switches, at a particular frequency and in complementary manner.
FIG. 2 shows one more detailed example of the circuit of FIG. 1.
In this example, the resonant tank 25 is in the form of an LLC resonant circuit, and it may be used to form a PFC stage. The circuit may thus be used as a PFC pre-regulator by having a controlled output voltage. It could also be used as a single stage LED driver by having a controlled output current.
The circuit comprises a mains input 10 which is followed by a rectifier bridge 12 having a high frequency filter capacitor 14 at the output. This generates the supply for the input terminal 2 (node B) of FIG. 1.
This example shows a converter with an isolated output. For this purpose, the converter comprises a primary-side circuit 16 and a secondary side 18. There is electrical isolation between the primary-side circuit 16 and the secondary side 18. A transformer comprising a primary coil 20 and a secondary coil 22 is provided for the isolation. The transformer has a magnetizing inductance 20 which also acts as one of the inductances of the series LLC resonant circuit. The LLC resonant circuit 25 has a second inductance 24, and a capacitance (formed as two capacitors 26 and 27 in this example).
In an LLC circuit, the inductances and capacitor may be in any series order. The inductor may comprise discrete components or it may be implemented as leakage inductances of the transformer.
The primary-side circuit 16 comprises the half-bridge 28, 30 and the resonant tank circuit 25.
The secondary side 18 has the rectifier 32 which is connected downstream of the secondary coil 22 and which can be formed, for example, by a first diode arrangement of diodes 32a and 32b and a second diode arrangement of diodes 34a and 34b. 
FIG. 2 shows a full-bridge rectifier and a single secondary coil which couples at its ends to the rectifier circuit. The low frequency (e.g. 100 Hz) storage capacitor CDC is connected between the outputs of the rectifier. The LED load or other output stage is represented in this figure by a resistor. In the case of an LED load, it comprises an LED or a plurality of LEDs.
The circuit shown in FIG. 2 may be used as an AC/DC PFC single stage converter or as PFC pre-regulator. FIG. 3 shows an alternative LLC half bridge topology, as a modification to FIG. 2 (and showing only the DC/DC conversion stage) in which the secondary coil 22 has a center tap and the full wave rectifier 32 is then implemented by two diodes. The LLC capacitor is also shown as a single component 35.
Half bridge resonant converters are used already in many applications like DC/AC converters for lighting applications, e.g. low- and high-pressure discharge lamp circuits, and DC/DC converters, e.g. DC power supplies and LED drivers.
The control block 31 drives the two power switches 28, 30 to conduct in an alternating sequence on and off, with a small non-conduction phase (dead time) used to avoid cross conduction of the power switches. A high gate drive signal turns on one switch and turns off the other switch and a low gate drive signal turns off the one switch and turns on the other switch. The advantage of using a resonant half bridge converter is that the current output current taken from the switching node X has no DC component and if this current has a phase lag, with respect to the switching node voltage VX, it can serve to discharge the parasitic output capacitance of the switch before it will be switched on.
The LLC circuit is susceptible to variations of its output voltage, which in a two stage circuit is the bus voltage for the output stage. In a typical design, the bus voltage is controlled to have a constant average value, and the 100 Hz (or 120 Hz) bus voltage ripple only depends on the bus (or storage) capacitor as well as on the converted power.
Designing the LLC-PFC pre-regulator for a given operating range in terms of power and AC-input voltages, taking into account the need for removing voltage ripple, may lead to compromises in terms of size and/or the efficiency at light load operation. The maximum mains voltage at the minimum load determines the maximum frequency operation point the converter has to be designed for.
Another issue is that in some low cost implementations of the circuit the input current waveform of the LLC circuit can deviate from the AC input voltage resulting in non-zero total harmonic distortion. This can be reduced by adapting the design (for example with an increased turns ratio). This can however lead to an overall decrease of efficiency due to an increase of the reactive current.
This invention relates to an improvement to the system in order to address these compromises in terms of efficiency, power factor and component size.
US 2012/106206 discloses a power supply having a single stage converter for performing power factor correction to reduce high-frequency harmonics in the input current and performing resonant conversion to achieve zero-voltage switching or zero-current switching for power conversion. The inventive single stage converter includes a switching circuit, a resonant circuit, a power control circuit, and a square wave generator. The switching circuit includes at least one switch and the resonant circuit includes a LLC resonant tank. The power control circuit includes a proportional differential circuit such as a power amplifier configured in a negative feedback topology, and the square wave generator is configured to generate driving signals based on the frequency modulation control signal generated by the comparison of the sensed input current and a user-defined power level input, thereby allowing the square wave generator to regulate the switching operation of the switching circuit.